Mark of Mystery

Sylwais i'r saeth hon ar bostyn ar gornel y stryd. Mae'r tipyn bach o baent melyn arni hi. Ro'n i'n meddwl ei bod hi'n rhywbeth milwrol, ond dw i wedi dysgu ei bod hi'n farc tirfesurydd. Ond, mae'r dirgelwch yw pam mae'n crafu ar bostyn ar gornel y stryd?

I noticed this arrow on q post on the street corner. There's a little bit of yellow paint on it. I thought that it was something military, but I've learned that it's a surveyor's mark. However, the mystery is why it is scratched on a post on the street corner?